Thirty percent of patients diagnosed with breast cancer ultimately develop metastatic disease and relapse in a process that can take years to decades. Tumor progression in the face of initially effective treatment represents a major clinical challenge. Tumor cells circumvent treatment via activation of adaptive and stress-induced signaling pathways. Metastasis and drug resistance occur in response to activation of a variety of stress-sensing signaling pathways whose cellular functions are orchestrated by the oxygen- responsive hypoxia inducible factors (HIFs). Yet, directly targeting HIFs is challenging as they are localized to the nucleus and regulate hundreds of genes. Most HIF inhibitors act indirectly rather than directly on HIF? stability/activity. To improve efficacy of treatments for breast cancr patients at risk of developing metastatic disease, driver genes acting downstream of HIFs must be identified and targeted. We have identified protein tyrosine kinase 6 (PTK6) as a major driver of oncogenic signaling in breast cancer. PTK6, also known as breast tumor kinase (Brk) is a non-receptor tyrosine kinase that is expressed in up to 86% of breast cancers but is very low or absent from normal breast tissues. Notably, increased PTK6 expression predicts poor outcome and mediates resistance to anti-cancer agents (ex. antibodies) aimed at cell-surface receptors. Forced expression of PTK6 confers hallmarks of cancer in vitro, including increased proliferation, pro-survival and migration. Recently, we found that PTK6 is rapidly upregulated in response to conditions that increase cellular stress and facilitate tumor progression, such as oxidative stress, low glucose, UV, and hypoxia. Robust PTK6 expression is also induced by corticosteroids, intracellular ligands for nuclear glucocorticoid receptors (GRs) and mediators of host stress responses. Notably, expression of GR is tightly associated with poor outcome in basal-type or triple-negative (ER-/PR-/Her2-) breast cancer (TNBC). TNBC cells also constitutively express high levels of HIF1? and HIF2?, transcription factor mediators of cellular responses to stress. Our most recent preliminary data suggest that GR/HIF complexes induce PTK6 in response to a variety of cellular stresses that may include chemotherapy. Once expressed, PTK6 activates the stress-associated protein kinases, p38 MAPK and ERK5, two emerging effectors of survival, chemo-resistance, EMT, and migratory/invasive behavior in breast cancer. These data collectively support a model in which cortisol/GR and/or tumor-associated stress mediate HIF-induced PTK6 expression, which then turns on stress-activated signaling pathways (p38 MAPK and ERK5), leading to increased cancer cell survival and the acquisition of migratory/invasive behaviors. In short, we hypothesize that inducible PTK6 confers chemo-resistance and metastatic behavior to tumor cells in vivo. Herein, we propose to define the requirements for PTK6 induction (Aim 1) and signal transduction (Aim 2) and test the role of PTK6 as a mediator of metastasis development and therapy-resistance in vivo using patient-derived xenograft (PDX) models of TNBC (Aim 3).